Human monoclonal autoantibodies against the immunodominant region on thyroid peroxidase: lack of cross-reactivity with related peroxidases or thyroglobulin and inability to inhibit thyroid peroxidase enzymatic activity.

Thyroid peroxidase (TPO) autoantibodies are heterogeneous and have been classified in terms of whether they cross-react with myeloperoxidase (MPO), lactoperoxidase (LPO), or thyroglobulin (Tg) as well as by whether they inhibit TPO enzymatic activity. Four human monoclonal TPO autoantibodies, generated using combinatorial immunoglobulin gene libraries and expressed as F(ab), have been used to investigate these properties of TPO autoantibodies. The binding of F(ab) WR1.7, TR1.8, TR1.9, and SP1.4 to 125I-labeled recombinant TPO was inhibited 50% by approximately 10(-10) mol/L unlabeled TPO, reflecting the high affinities of these F(ab) for TPO. In contrast, F(ab) binding to TPO was unaffected by human MPO (both native and reduced), bovine LPO, or human Tg at concentrations up to 10(-8) mol/L. Further, TPO enzymatic activity, measured by guiacol oxidation, was unaffected by preincubation with the four F(ab) individually or as a pool (each at 10(-8) mol/L). In conclusion, four human TPO monoclonal autoantibodies do not cross-react with related peroxidases or Tg, nor do they inhibit TPO enzymatic activity. These monoclonal immunoglobulin G class autoantibodies define the immunodominant region on TPO and represent about 85% of TPO autoantibodies in an individual patient's serum. Consequently, our data suggest that TPO autoantibodies that cross-react with MPO, LPO, or Tg, or inhibit TPO enzymatic activity are likely to bind outside the immunodominant region.